Microalloyed Steel Grades Research Articles

Formation Mechanism and Control Technology of Transverse Corner Cracks During Slab Continuous Casting of Microalloyed Steels

Microalloyed steel with Nb, V, Ti, and other alloying elements is one of the important steel grades in the steel industry. During continuous casting process, high occurrence rate of transverse corner cracks in slabs restricts the quality and efficiency of the production of microalloyed steel grades, and it has not been effectively solved for a long time. Herein, the research progress on the formation mechanism, influential factors, and typical control technologies of transverse corner cracks during slab continuous casting are reviewed. Considering high‐temperature solidification process of microalloyed steel, the control of growth of microstructures and the precipitation behavior of carbonitrides in slab corners are supposed to be the key to avoid transverse corner cracks during slab continuous casting.

Analysis of Recrystallization Kinetics Concerning the Experimental, Computational, and Empirical Evaluation of Critical Temperatures for Static Recrystallization in Nb, Ti, and V Microalloyed Steels

Recrystallization kinetics and two critical temperatures—the non-recrystallization temperature TNR and the static recrystallization critical temperature TSRCT—of five Nb, Ti, and V microalloyed steel grades are evaluated. The experimental examination is realized by employing isothermal double-hit compression tests and continuous hot torsion tests, both performed on a Gleeble® 3800 thermo-mechanical simulator. The experimental results are used for the critical assessment of predicted TNR using four empirical equations from the literature, and for the validation of simulated TNR and TSRCT. The thermokinetic computer simulations are realized using the mean-field microstructure modeling software MatCalc. Analysis shows that higher microalloying contents increase both critical temperatures, TNR and TSRCT, whereby the effect of recrystallization retardation of Nb is more pronounced than that of Ti or V. The most accurate reproduction of the experimental recrystallization behavior of the five examined steel grades is realized by the employed physics-based simulation approach.

Effect of Laser Shock Peening without Coating on Grain Size and Residual Stress Distribution in a Microalloyed Steel Grade

This study aimed to identify the optimal combination of wavelength and laser pulse density to achieve the optimal pulse pressure that can induce the maximum compressive residual stress at the subsurface of microalloyed steel. For this, laser shock peening without coating (LSPwC) was performed on microalloyed steel samples at the fundamental wavelength (1064 nm) with pulse densities of 3, 6, 9, and 12 GW/cm2 and at the second harmonic wavelength (532 nm) with pulse densities of 3, 6, and 9 GW/cm2. The residual stress distributions were studied to a depth of 500 µm in the laser-treated samples. Tensile residual stress was observed at the surface of laser-peened specimens in both wavelength conditions (1064 and 532 nm). The significant impartment of compressive residual stress across the depth was achieved at the fundamental wavelength (1064 nm). The maximum compressive residual stress was attained with a laser pulse density of 9 GW/cm2 in the 1064nm wavelength condition. The optical micrographic analysis in the subsurface regions of the LSPwC specimen at 1064 nm and 9 GW/cm2 shows evidence of a high degree of plastic deformation. Electron backscatter diffraction (EBSD) analysis shows that there is grain refinement due to plastic deformations in samples subjected to the fundamental wavelength. Microhardness distribution analysis across the subsurface region shows work-hardening effects in the laser-processed samples in the 1064 nm condition. This study also shows that there is an indication of a thermal softening effect in the samples treated with the 532 nm wavelength, and it is correlated with lower compressive residual stress across the depth.

Modelling stress relaxation after hot deformation: Microstructure-property relationships in Nb-bearing steels

Stress relaxation taking place after hot deformation is modelled by incorporating the simultaneous action of various physical phenomena. Dislocation recovery, recrystallization, and precipitation are all interrelated adopting new formulations. Dislocation recovery is approached through a vacancy-mediated dislocation climb approach. Recrystallization is considered to be meta-dynamic or static, and the effects of precipitate pinning and (interstitial and substitutional) solute drag are incorporated. The kinetics of precipitation nucleation, growth, and coarsening are also taken into account; precipitate nucleation in dislocation cells is assumed and thus the dislocation cell evolution is accounted for. The softening behaviour is thus tracked for 6 microalloyed steel grades and compared with 37 softening curves for a range of temperatures and prior deformation rates; the modelling describes the experiments with great accuracy. The application of the approach to additive manufacturing microstructural relaxation is discussed.

Fatigue Strength of Structural Steel DAN400 Plate Produced by Thermomechanical Treatment Technology with Accelerated Cooling

Fatigue properties of contemporary low-carbon micro-alloyed structural steel grades have not been studied sufficiently in spite of a large amount of available reference information. This slows down and hinders introduction of the latest generation of structural steel grades into production chains for manufacturers of heavy machinery and structures. According to multicycle fatigue test results for plate steel grade DAN400 36 mm thick with σy = 450 MPa, Cequ = 0.27%, and Pwi = 0.12% produced by thermomechanical treatment technology with accelerated cooling (TMTT + AC) the fatigue strength (σ–1, MPa) on a base of two million cycles, the index of the left-hand branch of the fatigue curve m = 1/α, and coefficients C and α in the equation for the left-hand branch of the fatigue curve σ–1 = С·N–α are determined. An experimental fatigue curve is constructed on double logarithmic coordinates for the test steel.

Study on Microstructure Transformation of High Strength Hot Rolled Steel by Using Finite Element Simulation

Advanced High-strength steels (AHSS) has widely application in automotive due to their high tensile strength and remarkable ductility. These good mechanical performances are strongly influenced by the processing and final microstructure. This paper performed Deformation Dilatometer and finite element simulation to study the effect of hot rolling parameters such as strain, cooling rate, and holding time at constant temperature on the microstructure formation of Nb-V low carbon microalloyed steel grade. It found that increasing deformation degree increased the volume fraction of ferrite, both of deformation dilatometer and finite element simulation give a similar trend of effects of hot rolling parameters on evolution of volume fraction of ferrite. These results give an insight for industrial application.

Correlative analysis of interaction between recrystallization and precipitation during sub-critical annealing of cold-rolled low-carbon V and Ti–V bearing microalloyed steels

In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti + grade containing Ti as microalloying element added into the base grade, and a Ti + Mn + grade containing additional Mn added into the Ti + grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti + grade, fine (<15 nm) (Ti,V)(C/N) particles retard the recrystallization kinetics. For the Ti+ and Ti + Mn + grades, solute drag effect of Mn solute atoms for dwell time longer than 2 min at annealing temperature of 800 °C is negligible.

СУЧАСНІ ТЕНДЕНЦІЇ ВИРОБНИЦТВА КОРОЗІЙНОСТІЙКОГО І ВОГНЕСТІЙКОГО НЕНАПРУЖЕНОГО АРМАТУРНОГО ПРОКАТУ

The aim of this work is to study modern methods of production of corrosion-resistant and fire-resistant stress-free reinforcing bars in the world. Non-stressed reinforcing bars are used as working, structural and assembly reinforcement in conventional building structures, as well as in various types of prestressed structures: from openwork prefabricated to huge monolithic hydraulic structures, therefore, its assortment produced by the domestic metallurgical industry in the range from 5,5 to 40 mm in the nominal diameter of a smooth profile and from 6,0 to 40 mm for a periodic profile. Recently, in the world, much attention is paid to the design and construction of buildings and structures with an improved complex of consumer properties, intended, in particular, for work in corrosive and fire hazardous environments. Such developments are not currently being conducted in Ukraine. In the world and in Europe, to increase corrosion resistance and fire resistance, as well as for the manufacture of seismically resistant reinforcing bars (δmax≥7,5 %), hot-rolled low-alloyed (additionally alloyed with V and Mo) and microalloyed steel grades without the use of heat treatment or cold-deformed (cоld stretched) from hot rolled strip of periodic profile. In GOST 34028-2016 for the countries of the Customs Union, it is clearly stated that for the reinforcement of prefabricated reinforced concrete structures and monolithic reinforced concrete, unstressed reinforcing bars in bars or coils with a nominal diameter of 4,0 to 40 mm are also used for the production of such rolled products, low-carbon and low-alloy steel grades are used, which microalloy V, Nb, Mo or add nitride-forming elements Al, Ti, V, Nb. In this case, either hot rolling or cold deformation or heat treatment in the flow of the rolling mill is used. An alternative way of producing corrosion-resistant and fire-resistant stress-free reinforcing bars in the world is the use of stainless steel as a material. Such profiled reinforcing bars are produced in the range of 3,0 to 50 mm by hot rolling or cold deformation. The use of modern technologies for the production of stainless steel reinforcing bars, starting with the addition of effective and high-quality alloying elements and additives in the production of liquid steel and effective methods of metal pressure treatment, which allows us to guarantee the material saving from corrosion and high temperatures in case of fire. A significant disadvantage of this rolled product is its rather high cost.

Recent advances in the understanding of the role of vanadium carbonitride precipitation to improve surface edge cracking on continuous casting of blooms

This study aims in combining the material properties and numerical modelling techniques through practical application to provide understanding of surface edge cracking caused by V(C, N) precipitation to enhance the yield of the continuous bloom caster at ArcelorMittal Duisburg. The investigation for this work is carried out on three different micro-alloyed steel grades; one of them being the most crack sensitive 20MnV6. A process model is used which calculates the solidification process of the strand to design an optimum cooling strategy. Therefore, two cooling patterns are employed for the steel grade 20MnV6. Reduction of area values of the steel grades 20MnV6, 27MnSiVS6 and 38MnSiVS5 evaluated from hot tensile tests using a Gleeble simulator have been used as an indication in assessing steel’s cracking behaviour. Further precipitates have been analysed by SEM at ArcelorMittal. These laboratory results suggest that precipitation kinetics of V(C, N) influences the crack sensitivity of the micro-alloyed steel. The software MatCalc® is used to simulate precipitation for process parameters of continuously cast blooms at Arcelor Mittal Duisburg as well as the parameters of the Gleeble experiments. From these simulations the Zener pinning force (ZPF), resulting from V(C, N) particles on grain boundaries, is evaluated which can be used as a measure for the crack sensitivity. Using the values of the ZPF it is possible to identify the set of casting parameters of the steel grade 20MnV6 which lead to the minimum crack intensity on the surface of the blooms. Moreover it is possible to make a ranking list with respect to the ductility drop occurring in the Gleeble experiments for the analysed steel grades. The proposed statement is an issue for a continuing investigation of precipitation modelling.

Effect of Temperature and Strain Rate of The Hot Deformation of V Microalloyed Steel on Flow Stress

Medium carbon micro-alloyed forging steels are employed in various automotive components. The impetus for the use of micro alloyed (MA) steels is cost reduction due to elimination of post-forging heat treatment. Compared to conventional quenched and tempered steels micro-alloyed steels can achieve similar or more superior properties simply by properly controlling the process parameters. Forging temperature, strain, strain rate and cooling rate are some of the important process parameters that influence the flow stress and final forging product quality. In the present study, hot compression test on a micro-alloyed steel grade 38MnSiVS5 were conducted on thermo-mechanical simulator (Gleeble-3500) to study the effect of temperature and strain rate on flow stress. The results indicate that the flow stress of 38MnSiVS5 steel is greatly affected by both deformation temperature and strain rate. Obtained true stress-true strain curves showed that the flow stress of the alloy increased by increasing the strain rate and decreasing the temperature, which can be represented in terms of an exponent type Zener-Hollomon equation. Finally, the constitutive equations for the flow behavior of 38MnSiVS5 microalloyed steel were determined.

Mathematical model for prediction of hydrogen pick-up of liquid steel during filling of a continuous casting tundish

Steels for several applications, such as steel cord for tires and heavy plates, present severe hydrogen restriction in their chemical composition. However, the hydrogen content of the liquid steel usually increases during tundish filling. This phenomenon, known as hydrogen pick-up, occurs due to direct steel contact with humidity present in the lining refractory, ladle sand and atmospheric air. The present work investigated by means of an analytical mathematical model the magnitude of hydrogen pick-up promoted by steel-air interaction. The relative importance of this mechanism was evaluated comparing the predictions with industrial data of hydrogen pick-up for a specific microalloyed steel grade. Results showed that air contamination can contribute to approximately 40% of the total hydrogen pick-up. An appropriate tundish inertization, which eliminates completely the air contamination, would be able to reduce approximately 1.5 ppm from the total hydrogen pick-up that happens during the beginning of the first heat of a sequence. As a consequence, it would be reduced more than 0.20 ppm of the total hydrogen pick-up in 36t of the steel cast.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

Elimination of Surface Defects in High Tensile Steel for Wheel Rim Application

One of the important applications of high tensile steel grades is in wheel rims by the automotive industry. High tensile steel facilitates vehicle weight reduction under the ‘Go Green’ initiative. Along with weight reduction, in recent times, the surface quality requirement of steels for wheel rim applications has also gained a significant focus, and in many cases, it is becoming the cause of rejection at the customers’ processing line. It is known that owing to chemistry requirements such high strength steel grades generally show peritectic behavior during solidification and are prone to surface defects. Tata Steel India produces several wheel rim grades of varying strength levels. In one such high manganese and micro-alloyed peritectic steel grade, the rejection by the customer on account of surface defects was a major concern. The defects consisted of typical longitudinal cracks of varying dimensions along with the unique defects in transverse direction named tear marks on the surface of 100-mm-thick plates rolled from 215-mm-thick slab. In subsequent stages, these plates were processed into rim profiles through hot working by the wheel customer. The paper highlights how systematic study led to arrive at root causes and helped redesign steel chemistry to ensure zero rejection for surface quality without affecting mechanical property requirements.

Influence of NbC-Precipitation on Hot Ductility in Microalloyed Steel - TEM Study and Thermokinetic Modeling

One of the main challenges during continuous casting of microalloyed steel is to avoid the formation of transverse surface cracks on the steel slabs. These cracks occur due to severe mechanical and thermal stresses in the strand during the straightening operation. The reason for this phenomenon is a ductility loss of austenite in a typical temperature range of 700°C - 1100°C. One of the main mechanisms reducing the ductility is the precipitation of carbides and nitrides. In this work, we correlate ductility loss and precipitation state accompanying two model cooling strategies in a microalloyed steel grade. Continuously cooled samples show a minimum of ductility at temperatures around 750°C. With increasing temperature, deformability recovers again to reach full ductility again at 950°C. In contrast, samples treated with a fast intermediate cooling and reheating show constant low ductility in this entire temperature range. A transmission electron microscopy (TEM) investigation shows nanometer-sized NbC precipitates in the low ductility states. In contrast, in the samples with high ductility, larger NbC precipitates with lower number densities are observed. The experimental results show a good accordance with corresponding precipitation kinetics simulations carried out with the MatCalc software package.

Synchrotron Investigation on the Precipitation Behaviour of Niobium Microalloyed Steel

The precipitation and dissolution behavior of niobium carbo-nitrides is of particular interest for many technical applications. Niobium-microalloyed high strength low alloy (HSLA) steels are widely used in civil construction, automobile and line pipe applications. These steels rely on thermomechanical processing. In this context, coupled processes like thin slab casting and thermomechanical rolling of microalloyed steel grades require most precise information on the precipitation state at the individual processing steps. Reasonable equations for the solubility product at thermal equilibrium can be taken from literature but kinetics is largely unknown. Conventional X-ray technology is not able to detect small volume fractions below 0.1% of nanoscale precipitates. Investigation of nanoscale niobium precipitates by transmission electron microscopy (TEM) analysis or chemical extraction methods is common practice. However, TEM suffers from statistical relevance and chemical extraction will not give information on particle distribution and orientation. Investigation by high energy synchrotron X-ray of about 100 keV offers statistical relevance as volumes of several cubic millimeters are regarded. This large reflecting sample volume allows to detect nanometer-sized particles and provides very high angular resolution leading to an exact determination of the reflection peaks. The wavelength of around 0.12 Å is able to analyze nanometer-sized particles. Due to the high energy of the applied synchrotron radiation, precipitation and dissolution reactions could be observed during thermal treatment inside a soaking furnace. The results establish this technology for analysis of nanoscale niobium carbo-nitride precipitates

Some Metallurgical Issues Concerning Austenite Conditioning in Nb-Ti and Nb-Mo Microalloyed Steels Processed by Near-Net-Shape Casting and Direct Rolling Technologies

As thin slab direct rolling technologies are moving to the production of higher quality steel grades, chemical compositions based on Nb-Ti and Nb-Mo become a good option. However, with the use of multiple microalloying additions, the as-cast austenite conditioning becomes more complex. This paper analyzes some of the microstructural features that should be taken into account during the as-cast austenite conditioning in Nb-Ti and Nb-Mo microalloyed steel grades. In the case of Nb-Ti grades, it has been observed that the process parameters during solidification and post-solidification steps affect the austenite evolution during hot rolling. This is due to the differences in the size and volume fraction of TiN particles that can be formed. Fine TiN precipitates have been shown to be able to delay recrystallization kinetics. Moreover, the solute drag effect of Ti cannot be ignored in the case of hyperstoichiometric Ti/N ratios. It is observed that Nb-Ti grades tend to have lower non-recrystallization temperatures compared to Nb grades, which means that pancaking of the austenite is more difficult for these steels. The opposite is observed for the Nb-Mo grades, although in both cases the behavior is affected by the nominal content of Nb.

Long-range diffusion of H in the presence of traps in a microalloyed steel

In the present work, we apply a new model for trapping of interstitial elements at various types of atomic traps to investigate the H-trapping efficiency of different kinds of precipitates in a microalloyed steel grade. The model is briefly introduced and reviewed. The trapping strength representative for the different stages of the precipitation process is evaluated, as well as the influence of size and shape of the particles on the trapping capacity. The significant impact of the applied trapping energies between precipitate interface and H-atoms is discussed. On basis of the present work, the considerable scatter of experimental H-diffusion data at low temperatures is straightforwardly explained and quantitatively reproduced in the simulations.

Effect of Different Rolling Schedules on the Mechanical Properties and Microstructure of C Mn (V-Nb-Ti) Pipeline Steel

High strength low-alloy (HSLA) steels have been used for the production of welded pipes for more than 30 years. However, the alloy design of pipeline grades is being continuously modified and the process technology optimized because of increasing demand of high strength toughness combination requirement of pipeline steels. The HSLA steels have demonstrated superior mechanical properties through controlled rolling. In the present investigation, the effects of processing parameters, such as finish rolling temperature, rolling reduction and cooling rate, on the final microstructure and mechanical properties of a grade X70 type HSLA steel has been studied by tensile and charpy impact tests and optical microscopy. To yield better mechanical properties of X70 microalloyed steel grade, the optimization of the rolling process in the laboratory experiment and rolling mill has been carried out.

Deformation Behavior of Micro-alloyed Steel by Using Thermo Mechanical Simulator and Finite Element Method

Micro-alloyed (MA) forging steels are becoming increasingly important economic alternative to the traditional quenched and tempered steels. MA steels do not require heat treatment after they are shaped into parts, as the required mechanical properties are obtained directly at the end of the process and save the cost and energy by reducing the number of operations. Also, these steels have achieved very good characteristics of toughness and weldability. These beneficial properties have been achieved by a careful control of chemical composition and by adopting suitably controlled thermo- mechanical processes. In this study, hot compression test on a medium carbon vanadium micro-alloyed steel grade 38MnVS6 has been performed on Gleeble-Thermo mechanical Simulator (TMS) in order to study the effect of forging temperature and strain rate on flow stress. Also the proposed process has been simulated using a forging process simulation software tool, DEFORMTM 3D V6.1.